Crystal resonators are known to comprise a rectangular, piezoelectric crystal substrate having two substantially planar, parallel major surfaces on opposite sides of the crystal substrate. Electrodes are formed on the major surfaces such that a voltage applied across the electrodes causes the crystal substrate to resonate. In order for crystal resonators to be surface-mountable, provisions must be made such that electrical connections can be established between both the top and bottom electrodes and a mounting substrate. It is often difficult and/or costly, however, to establish electrical connections (typically through the use of solder or another electrically conductive adhesive) between the top electrode and the mounting substrate.
One solution to this problem is to use an automated device, such as a robotic arm, to place a "top dot" of conductive adhesive on the crystal resonator. This operation wraps conductive adhesive from a conductive pad on the mounting substrate, around the crystal substrate, to the top electrode such that a reliable electrical connection is created. Automated devices capable of such delicate operations are often very expensive, costing as much as one million dollars apiece, thus adding substantially to the overall cost of mounting each crystal resonator.
An alternative to this method is to fabricate the crystal resonator in such a way that a specialized automated device is not required to establish electrical connection with the top electrode. It is known, for instance, to place holes in the crystal substrate such that when the crystal resonator is mechanically placed on the mounting substrate, conductive adhesive already positioned on the conductive pad is allowed to flow through the crystal substrate and establish an electrical connection with the top electrode. A limitation of this method, however, is that the differing coefficients of expansion of the crystal substrate and the conductive adhesive can cause undo stresses on the crystal substrate when environmental conditions vary. These stresses, in turn, can adversely affect the performance of the crystal resonator. Therefore, a need exists for a surface-mountable crystal resonator that eliminates the need for additional automated mounting devices and overcomes the limitations of prior art solutions.